1. Field of the Invention
The present invention relates to a barium titanate-based semiconducting ceramic (PTC thermistor) which has a positive temperature coefficient (PTC) and which is used as in a low-temperature heating element and in an automatic demagnetizer for a color TV.
2. Description of the Background
It is known that barium titanate, BaTiO.sub.3, which has a perovskite type crystal structure is transformed by the addition of a minute amount of a semiconductor-forming agent such as a rare earth metal, niobium or antimony into a semiconductor, so as to manifest the PTC (positive temperature coefficient) phenomenon in which the magnitude of resistance abruptly increases at a temperature no less than the Curie point.
In recent years, studies have been actively carried out to elucidate this peculiar phenomenon and to find practical applications. The semiconducting ceramic compositions of the barium titanate type have found utility in various heating elements, and switching elements, sensors, and automatic demagnetizers for color TV.
The semiconducting ceramic of the barium titanate type is required to have high voltage resistance in order to ensure reliability as a product element. Further, for full operability as a product element, it is desirable to have appropriate room-temperature resistivity (for example, a resistivity .rho.25 in the range of 10 to 400 .OMEGA..multidot.cm) which is neither too large nor too small.
In order to increase voltage resistance, various proposals such as those as disclosed in JP-A-4-338601 and JP-A-7-335404, have been heretofore offered. Specifically, JP-A-4-338601 proposes that the voltage resistance be improved by setting the ratio, In/Is, of the X-ray diffraction intensity, In, of the Ba.sub.2 TiSi.sub.2 O.sub.8 (111) plane in the central part of the sinter of a barium titanate semiconductor to the X-ray diffraction intensity, Is, of the Ba.sub.2 TiSi.sub.2 O.sub.8 (111) plane in the surface part of the sinter to be not less than 7. JP-A-7-335404 proposes that a barium titanate type semiconducting ceramic having a high resistance temperature coefficient and a high voltage resistance be obtained by causing a main component of barium titanate to contain excess TiO.sub.2 in the range of 0.5 to 3 mol %, relative to 1 mol of the main component.
The barium titanate type semiconducting ceramics disclosed in JP-A-4-338601 and JP-A-7-335404, mentioned above, both show source improvement in the voltage resistance, but is still unsatisfactory. Furthermore, the ceramic which is disclosed in JP-A-4-338601 has a room temperature resistance of about 0.85 to 0.87 which is too small. Besides, it is difficult to locate the point for measuring the X-ray diffraction intensity depending on the shape of an element, so that it is very difficult to produce the element which satisfies the requirements proposed in the publication. Further, the ceramic disclosed in JP-A-7-335404 is not suitable for the use described hereinbelow in accordance with the present invention because it has a room temperature resistance which is too large and which, thus, is almost an insulator.